Method and system of evaluating distribution of lattice strain on crystal material

ABSTRACT

A crystal material lattice strain evaluation method includes illuminating a sample having a crystal structure with an electron beam in a zone axis direction, and selectively detecting a certain diffracted wave diffracted in a certain direction among a plurality of diffracted waves diffracted by the sample. The method further includes repeating the illuminating step and the selectively detecting step while scanning the sample, and obtaining a strain distribution image in a direction corresponding to the certain diffracted wave from diffraction intensity at each point of the sample.

TECHNICAL FIELD REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of the prioritiesof Japanese patent application No. 2011-112618, filed on May 19, 2011and Japanese patent application No. 2012-034676, filed on Feb. 21, 2012the disclosure of which is incorporated herein in its entirety byreference thereto.

The present invention relates to a method and a system of evaluating adistribution of lattice strain on crystal material. In particular, itrelates to a method and a system of evaluating a distribution of latticestrain on crystal material used in a semiconductor device or the like byusing electron beam diffraction.

BACKGROUND DESCRIPTION OF THE RELATED ART

When an LSI device is manufactured, stress generated by use of varioustypes of material causes lattice strain on a crystal structure used in asemiconductor device. Such lattice strain is one of the importantphysical quantities that exhibit crystal material properties. The stressand lattice strain change, depending on the difference in mechanicalphysical properties of various types of material used in LSI deviceprocesses or depending on heat treatment used in processes. The latticestrain is a cause of a crystal defect or the like, resulting in devicefailure. In addition, in recent years, for example, attempts are beingmade to improve the electron and hole mobility of silicon, by using thelattice strain. Namely, attempts are being made to improve physicalproperties of crystal material by actively utilizing the lattice strain.Thus, if the lattice strain is utilized properly, improvement in deviceperformance can be expected. However, if the lattice strain is notcontrolled properly, a crystal defect leading to device malfunction iscaused. Therefore, evaluation of the stress and lattice strain oncrystal material and optimization of process conditions are essential inthe development of LSI devices.

Conventionally, the stress and lattice strain on crystal material suchas in a semiconductor device have been evaluated by X-raydiffractometry, Raman spectroscopy, or the like. However, recentreduction in device size is making these conventional stress and latticestrain evaluation methods insufficient in spatial resolution, and it isbecoming more difficult to obtain sufficient results. Therefore, aconvergent-beam electron diffraction (CBED) method and a nano-beamelectron diffraction (NBD) method are being proposed as methods using anelectron beam and evaluating localized stress and lattice strain.

For example, Patent Document 1 discloses an apparatus and a method usingthe CBED method and evaluating strain on crystal material. In addition,Non-Patent Document 1 discloses using the NBD method and evaluatingstrain on an SOI MOSFET. In addition, Patent Documents 2 and 3 disclosea method of using a diffraction contrast and two-dimensionallyevaluating lattice strain instantly. Non-Patent Document 2 discloses astrain evaluation method using thermal diffuse scattering electronintensity. Patent Document 4 discloses a semiconductor device having atrench-gate transistor as an example of a minute device structurerequiring localized stress and lattice strain evaluation.

Patent Document 1:

-   Japanese Patent Kokai Publication No. JP-H06-36729A

Patent Document 2:

-   Japanese Patent Kokai Publication No. JP2004-93263A

Patent Document 3:

-   Japanese Patent Kokai Publication No. JP2006-242914A

Patent Document 4:

-   Japanese Patent Kokai Publication No. JP2007-123551A, which    corresponds to US Patent Application Publication No.    US2007/0096204A1.

Non-Patent Document 1:

-   K. Usuda et al., “Strain characterization in SOI and strained-Si on    SGOI MOSFET channel using nano-beam electron diffraction (NBD)”,    Materials Science and Engineering B124-125 (2005), p. 143 to 147.

Non-Patent Document 2:

-   N. Nakanishi et al., “Strain Mapping Technique for Performance    Improvement of Strained MOSFETs with Scanning Transmission Electron    Microscopy”, IEDM2008, p. 431 to 434.

SUMMARY

The disclosure of the above Patent Documents and Non-patent Documentsare incorporated herein in their entirety by reference thereto. Thefollowing analysis is given by the present invention. Since the aboveCBED and NBD methods are used for evaluation on one-dimensional points,it is insufficient to discuss the influence caused by lattice strain ondevice characteristics. Even with such evaluation method based on theCBED and NBD methods, in principle, it is possible to set manyevaluation points and obtain a two-dimensional distribution. However,since the analysis method requires complicated and time-consumingoperations, such method is not effective. In addition, these evaluationmethods are not established as evaluation apparatuses. The evaluationmethods are merely known as application examples of a transmissionelectron microscope. Namely, skilled techniques are required to obtainreliable results.

In addition, based on the evaluation methods disclosed in PatentDocuments 2 and 3 and Non-Patent Document 2, lattice strain in a varietyof directions is added and measured, it is insufficient to discuss theinfluence caused by lattice strain in a current direction. Generally,LSI devices are planarly formed on a silicon semiconductor wafer or thelike and are designed so that current flows in one direction. Thus, todiscuss the influence caused by strain on device characteristics, it isnecessary to evaluate strain, in view of directional components, such asa strain distribution, principal strain, and shear lattice strain ineach of various directions.

According to a first aspect of the present invention, there is provideda method of evaluating a distribution of lattice strain on crystalmaterial. The method comprises illuminating a sample having a crystalstructure with an electron beam in a zone axis direction (termed“illuminating step”), and selectively detecting a certain diffractedwave diffracted in a certain direction among a plurality of diffractedwaves diffracted by the sample (termed “selectively detecting step”).The method further comprises repeating the illuminating step and theselectively detecting step while scanning the sample, and obtaining astrain distribution image in a direction corresponding to the certaindiffracted wave from diffraction intensity at each point of the sample.

According to a second aspect of the present invention, there is provideda system of evaluating a distribution of lattice strain on crystalmaterial. The system comprises a scanning transmission electronmicroscope that illuminates a sample with an electron beam, scans thesample, and detects a diffracted wave transmitted or diffracted by thesample. The system further comprises a strain distribution imageextraction unit that selects a certain diffracted wave among diffractedwaves transmitted or diffracted by the sample and obtains a straindistribution image.

According to a third aspect of the present invention, there is provideda non-transitory computer-readable recording medium storing a computerprogram used in an evaluation system. The evaluation system comprises ascanning transmission electron microscope and a computer that controlsthe scanning transmission electron microscope and processes measurementdata obtained by the scanning transmission electron microscope.

The computer program causes the computer to execute processes of:controlling the scanning transmission electron microscope so that asample having a crystal structure is illuminated with an electron beamand a diffracted wave transmitted or diffracted by the sample isdetected; and scanning the sample, selecting a certain diffracted waveamong diffracted waves transmitted or diffracted by the sample, andobtaining a strain distribution image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an overall configuration of asystem of evaluating a distribution of lattice strain on crystalmaterial according to an exemplary embodiment of the present disclosure.

FIG. 2 is a flow chart illustrating a method of evaluating adistribution of lattice strain on crystal material according to anexemplary embodiment of the present disclosure.

FIG. 3A illustrates the direction in which an electron beam is incidenton crystal of a semiconductor device as an evaluation sample and FIG. 3Billustrates directions of diffracted waves.

FIGS. 4A and 4B illustrate a principle of obtaining a straindistribution image from a diffraction image of a certain diffractedwave: FIG. 4A illustrates a relationship among a reciprocal latticepoint, an Ewald sphere, and an excitation error; and FIG. 4B illustratesthe relationship between a reciprocal lattice point and an excitationerror when lattice strain is caused.

FIGS. 5A to 5C illustrate a diffracted wave, a diffracted wave intensityimage, and a stress analysis diagram, respectively, used for analyzingstrain in the X direction; and FIGS. 5D to 5F illustrate a diffractedwave, a diffracted wave intensity image, and a stress analysis diagram,respectively, used for analyzing strain in the Y direction.

FIG. 6A illustrates a diffracted wave intensity image; FIG. 6Billustrates a conversion scale between the diffracted wave intensity andthe lattice strain magnitude; and FIG. 6C illustrates a calibrationcurve of the diffracted wave intensity and the lattice strain obtainedby the NBD method.

FIG. 7 is a flow chart illustrating a method of analyzing lattice strainfrom a lattice strain distribution.

FIG. 8A is a plan view when an STI interface is formed perpendicular tothe crystal axis of an active region; FIG. 8B is an enlarged viewillustrating the stress direction; FIG. 8C is a plan view when an STIinterface is formed with a taper angle with respect to the crystal axis;and FIG. 8D is an enlarged view illustrating the stress direction.

FIGS. 9A and 9B illustrate a method of determining a stress source fromthe magnitude and direction of principal strain at each point.

FIG. 10A illustrates the direction in which stress is caused and FIG.10B illustrates the direction in which a reciprocal lattice point ismoved when perpendicular stress is caused on a sample.

FIG. 11A illustrates the direction in which stress is caused and FIG.11B illustrates the direction in which a reciprocal lattice point ismoved when complex stress is caused on a sample.

FIGS. 12A and 12B illustrate shapes of Ewald spheres formed in areciprocal space when the electron beam incident on a sample has a largeconvergent angle and a small convergent angle, respectively.

FIG. 13 is a block diagram illustrating an overall configuration of asystem of evaluating a distribution of lattice strain on crystalmaterial according to a fourth exemplary embodiment.

FIG. 14A is a plan view of a rectangular aperture according to thefourth exemplary embodiment and FIG. 14B is a perspective viewillustrating an electron beam being incident on a sample.

FIG. 15A illustrates the relationship between a cross section (YZcross-section) of the electron beam incident in the longitudinaldirection of the rectangular aperture and reciprocal lattice points and

FIG. 15B illustrates the relationship between a cross section (XYcross-section) of the electron beam and the reciprocal lattice pointsaccording to the fourth exemplary embodiment.

FIG. 16 is a plan view illustrating another rectangular apertureaccording to the fourth exemplary embodiment.

FIG. 17 is a flow chart illustrating a method of evaluating adistribution of lattice strain on crystal material according to thefourth exemplary embodiment.

PREFERRED MODES Exemplary Embodiments

A summary of an exemplary embodiment of the present disclosure will bedescribed. For example, by using a scanning transmission electronmicroscope as illustrated FIG. 1, if an electron beam is emitted to beincident on a thin semiconductor device sample as illustrated in FIG.3A, a transmitted wave and diffracted waves as illustrated in FIG. 3Bcan be observed. As illustrated in FIGS. 5A to 5C and FIGS. 5D to 5F,from diffracted waves diffracted in different directions, a crystalstrain distribution image corresponding to each of the diffractiondirections can be obtained. In addition, by using a known method such asthe NBD method to quantify strain distribution images, a shear straindistribution and a principal strain distribution can be grasped fromquantified strain distribution images in a plurality of directions.Thus, a cause of strain can be determined, and the possibility ofoccurrence of a crystal defect can be predicted. The drawings referredto by way of symbols in this summary are merely used as examples tofacilitate understanding of the present disclosure. Therefore, thepresent invention is not limited by the modes illustrated by thedrawings.

Next, each of the exemplary embodiments will be described in detail withreference to the drawings.

First Exemplary Embodiment

FIG. 1 is a block diagram illustrating an overall configuration of asystem 10 of evaluating a distribution of lattice strain on crystalmaterial according to a first exemplary embodiment. The evaluationsystem 10 in FIG. 1 includes: a scanning transmission electronmicroscope 100; and a calculation processing apparatus 200 controllingthe electron microscope 100 and processing measurement data obtained bythe electron microscope 100.

The electron microscope 100 includes an electron beam source 110outputting an electron beam used for observing an evaluation sample 300;an illuminating-system lens apparatus 120 using the electron beamoutputted from the electron beam source 110 to illuminate the evaluationsample 300; and an imaging-system lens apparatus 150 functioning as anobjective lens focusing an electron beam on a minute spot region of theevaluation sample 300. An electron beam transmitted by the evaluationsample 300 or an electron beam diffracted in the forward direction bythe evaluation sample 300 is focused on an electron beam detector 190 bya projecting-system lens apparatus 160.

In addition, the electron microscope 100 includes a scanning coil 130causing the electron beam emitted from the electron beam source 110 toscan the evaluation sample 300. The scanning coil 130 controls theelectron beam to scan the evaluation sample 300. In addition, theelectron microscope 100 includes a deflecting coil 170 selecting acertain one of the electron beams transmitted by the evaluation sample300 or diffracted in the forward direction by the evaluation sample 300and focusing the certain electron beam on the electron beam detector190. In addition, the electron microscope 100 includes a scanning coiland lens control apparatus 180 controlling the scanning coil 130, theilluminating-system lens apparatus 120, the imaging-system lensapparatus 150, the projecting-system lens apparatus 160, and thedeflecting coil 170.

In addition, the electron microscope 100 includes a sample orientationcontrol apparatus 140 controlling the orientation of the evaluationsample 300, to align the crystal axis direction of the evaluation sample300 with the electron beam illumination direction. The sampleorientation control apparatus 140 executes fine-tuning of the directionof the evaluation sample 300, to align the crystal axis direction of theevaluation sample 300 with the electron beam illumination direction.

The sample orientation control apparatus 140 and the scanning coil andlens control apparatus 180 are connected to the calculation processingapparatus 200 and are controlled by a control unit 210 of thecalculation processing apparatus 200. The electron beam detector 190 isalso connected to the calculation processing apparatus 200 processingmeasurement data detected by the electron beam detector 190.

The calculation processing apparatus 200 is connected to an inputapparatus 270, a storage apparatus 280, and a display apparatus 290. Theinput apparatus 270 includes an operation interface such as a keyboardand a mouse, so that an operator can control an overall operation of theevaluation system 10. The storage apparatus 280 can store programs forcontrolling measurement data and an overall operation of the evaluationsystem 10 and for analyzing measurement data. The display apparatus 290can display measurement data detected by the electron beam detector 190as image data and can display results obtained by evaluation andanalysis executed by the calculation processing apparatus 200.

The calculation processing apparatus 200 includes, as incorporatedfunctions, the control unit 210 controlling operations of the electronmicroscope 100, a strain distribution image extraction unit 221extracting a strain distribution image from data detected by theelectron beam detector 190, a strain quantification unit 230 using theNBD method or the like to quantify the magnitude of lattice strain basedon a diffraction image position observed by the electron beam detector190, and a stress analysis unit 240 analyzing stress caused on each ofthe regions of the evaluation sample 300 based on quantified straindistribution images in a plurality of directions or the like.

FIG. 2 is a flow chart illustrating a method of evaluating adistribution of lattice strain on crystal material. Next, a method ofevaluating lattice strain on a semiconductor device will be describedwith reference to FIG. 2. First, in step S1 in FIG. 2, an FIB (FocusedIon Beam) method or the like is used to form a sectional TEM(Transmission Electron Microscope) sample of a semiconductor devicehaving a uniform thickness. In this step, when the sample is formed, itis desirable that the sample should be adjusted to have a thickness of200 nm or less and that the sample should not have different structuraldistributions in the thickness direction in accordance with dimensionsof the semiconductor device as much as possible (namely, it is desirablethat the sample should be formed to have an uniform structure in thethickness direction).

FIG. 3A illustrates an evaluation sample of a semiconductor device. Thesemiconductor device in FIG. 3A is a semiconductor device on which ageneral silicon-crystal MOSFET is formed. As illustrated in FIG. 3A, thesectional TEM sample of the semiconductor device is formed so that theX-axis and Y-axis directions of the single-crystal silicon of an activeregion 310 are <110> and <001>, respectively. The active region 310 isformed in the middle of the semiconductor device in the X-axisdirection, and a shallow trench isolation 320 is formed on either sideof the active region 310 in the X-axis direction. In addition, a gate350 is formed on the surface of the semiconductor device (positivedirection in the Y-axis direction), and a contact plug 340 connected tothe active region 310 is formed on either side of the gate 350 in theX-axis direction.

Next, the sample formed in step S1 is placed at a position correspondingto the position of the evaluation sample 300 in the (scanningtransmission) electron microscope 100 of the evaluation system 10 inFIG. 1. In addition, the sample orientation control apparatus 140 isused to control the orientation of the evaluation sample 300 so that theelectron beam is incident on the evaluation sample 300 along the zoneaxis thereof (step S2 in FIG. 2). In this example, the zone axis alongwhich the electron beam is incident is set to be in the Si <110>direction. In FIG. 3A, the X-axis and the zone axis directions in whichthe electron beam is incident are perpendicular to each other. In viewof the crystalline symmetry, both the directions are denoted by <110>.Namely, in FIG. 3A, the sample orientation control apparatus 140controls the orientation of the evaluation sample 300 so that theelectron beam is incident in the direction perpendicular to both of theX-axis and the Y-axis.

Next, the deflecting coil 170 is adjusted so that a certain diffractedwave is captured by the electron beam detector 190 among an electronicdiffraction pattern which is formed downstream of the evaluation sample300 (in the traveling direction of the electron beam) when the electronbeam is focused on the evaluation sample 300 (step S3 in FIG. 2).Namely, if the electron beam is emitted to be incident on the evaluationsample 300 in the direction perpendicular to the X-axis <110> and theY-axis <001> in FIG. 3A, a transmitted wave and a plurality ofdiffracted waves such as diffracted waves 002, 004, 111, and 220 arefocused near the electron beam detector 190 located downstream of theevaluation sample 300 (in the traveling direction of the electron beam),as illustrated in FIG. 3B. The crystal plane of the active region 310and the diffracted waves correspond to each other. For example, in stepS3, the deflecting coil 170 is controlled so that the electron beamdetector 190 captures the diffracted wave 220 when lattice strain in theX direction is evaluated and the diffracted wave 004 or 002 when latticestrain in the Y direction is evaluated. Thus, among the transmitted waveand the diffracted waves, only a certain diffracted wave is captured bythe electron beam detector 190.

While the deflecting coil 170 is fixed so that only a certain diffractedwave is captured by the electron beam detector 190, the scanning coil130 is controlled to cause the electron beam to scan the evaluationsample 300. Consequently, a contrast image corresponding to a straindistribution in a certain direction (the X <110> direction or the Y<001> direction in FIG. 3A) can be obtained (in steps S4 and S5 in FIG.2).

If no quantitative evaluation is necessary for the lattice strain (No instep S6), a strain distribution image in each direction is acquired andthe process is ended (step S7). If a quantitative evaluation isnecessary for the lattice strain (Yes in step S6), the strain amount isquantified by the NBD method or the like and a quantitative straindistribution image is acquired (in steps S8 and S9). For example, asillustrated in FIG. 6C, the NBD method can be used to measure a fewpoints of diffracted wave intensity and lattice strain, and themeasurement values can be processed by linear regression to obtain acalibration curve. Based on this calibration curve, by displaying aconversion scale between the diffracted wave intensity and the latticestrain magnitude as illustrated in FIG. 6B along with a diffracted waveintensity image as illustrated in FIG. 6A, a quantified straindistribution in a certain direction can be displayed.

In FIG. 3A, the TEM sample is formed so that the zone axis in theelectron beam incident direction is set to be in the Si <110> direction.However, even if the zone axis in the electron beam incident directionmay be set in the Si <100> direction, a strain distribution based on adiffracted wave can be evaluated. Namely, a strain distribution in anarbitrary crystal axis direction can be evaluated.

Next, a principle of acquiring a contrast image corresponding to strainin accordance with the above method will be described with reference toFIGS. 4A and 4B. FIG. 4A illustrates reciprocal lattice points, anincident electron beam, and an Ewald sphere in a reciprocal latticespace. Based on Bragg's law, an electron beam incident on crystal isdiffracted from the incident angle by 2θ. The intensity is determined byan excitation error Sg, which is the distance between a reciprocallattice point and the Ewald sphere illustrated in FIG. 4A. Theexcitation error Sg is represented by expression 1, assuming that grepresents a reciprocal lattice vector, k: a wave vector, d: latticespacing, a: a lattice constant, h,k,l: a lattice plane index, and λ: anelectron beam wavelength.

$\begin{matrix}{{S_{g} \propto \frac{{g}^{2}}{k}}{{g} = {\frac{1}{d_{hkl}} = \frac{\sqrt{h^{2} + k^{2} + l^{2}}}{a}}}{{k} = \frac{1}{\lambda}}} & \left\lbrack {{expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

If stress is caused on the crystal material and if the lattice strain iscaused, a reciprocal lattice point is moved horizontally as illustratedin FIG. 4B. Accordingly, if the reciprocal lattice point is moved, sincethe excitation error Sg is changed, the diffraction intensity is alsochanged. The excitation error Sg when the reciprocal lattice point ismoved can be represented by expression 2.

$\begin{matrix}{S_{g} \propto \frac{{{g + {d\; g}}}^{2}}{k}} & \left\lbrack {{expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The electron beam wavelength used by a general electron microscope isapproximately 0.0197 [Å] (when the acceleration voltage is 300 kV).Since the lattice strain evaluated with respect to the Ewald sphereradius given by the reciprocal of the wavelength is approximately a few% at most, the relationship between the excitation error Sg and thelattice strain can suitably be expressed by straight-line approximation.In addition, while there is an extinction distance ξg as a factoraffecting the diffraction intensity, this is a parameter that is mainlydependent on the sample thickness and that is changed depending on thesample thickness or incident intensity. In the present exemplaryembodiment, a sample having a sufficiently uniform thickness can beformed by the FIB method and the sample can be scanned in a constantincident direction. Thus, the diffraction intensity is not affected.

Since a minute device having a complex structure has a complex latticestrain distribution, use of the NBD method or the CBED method requiresmuch time for detailed evaluation of a strain distribution. However, byusing the method according to the first exemplary embodiment, a straindistribution image can be acquired instantly.

Second Exemplary Embodiment

In the first exemplary embodiment, outputting a quantitative straindistribution image in each direction is described. In a second exemplaryembodiment, based on the output results according to the first exemplaryembodiment, a principal strain distribution and a shear straindistribution in an arbitrary direction are outputted to grasp the causeof crystal strain and to predict the possibility of occurrence of acrystal defect. FIG. 7 is a flow chart illustrating a process procedureaccording to the second exemplary embodiment.

Before the second exemplary embodiment is described, evaluation examplesof lattice strain caused when a shallow trench isolation (STI) generallyused in a silicon LSI device causes stress to an active region will bedescribed with reference to FIGS. 8A to 8D. FIG. 8A is a plan view whenan STI interface is formed in alignment with the crystal orientation inan active region. FIG. 8B is an enlarged view of a portion near theinterface. FIG. 8C is a plan view when an STI interface is formed with ataper angle with respect to the crystal structure in an active region.FIG. 8D is an enlarged view of a portion near the interface.

As illustrated in FIGS. 8A and 8B, if an STI interface is formed inalignment with the crystal orientation in an active region, generally,stress is caused on the crystal structure in the active region in thedirections perpendicular thereto. However, as illustrated in FIGS. 8Cand 8D, if an STI interface is formed with a taper angle with respect tothe crystal structure in an active region, the active region issubjected to not only the strain in the X and Y directions but alsoshear strain in the X and Y directions. It is known that such shearstrain in a semiconductor device is a cause of a crystal defect. Thus,it is necessary that a manufacturing or design process should becontrolled so that such shear strain does not cause a crystal defect.

However, by evaluating strain only in the X and Y directions, such shearstrain cannot be determined. In addition, since strain in a directionperpendicular to an STI interface exhibits a maximum level, if thedirection of a principal strain, which is the strain exhibiting amaximum level, can be determined, a stress source causing the strain canbe determined, making it easier to provide feedback to the semiconductormanufacturing process. Next, an analysis method according to the secondexemplary embodiment will be described. This method analyzes shearstrain and principal strain, based on lattice strain distribution imagesin different directions obtained under different imaging conditionsaccording to the first exemplary embodiment.

In the second exemplary embodiment, as in the first exemplaryembodiment, a TEM sample formed by the FIB method is used. Thus, thestress component in the sample depth direction (in the direction thatthe electron beam is transmitted) is set to be zero through stressrelaxation during the FIB processing. Consequently, two-dimensionalstress approximation is possible. Hereinafter, lattice strain in atwo-dimensional stress state will be described.

In FIG. 3B, strain distributions (ε_(xx)), and (ε_(yy)), and (ε_(θ),θ=35.3 degrees) in the X direction, the Y direction, and the 35.3-degreedirection can be obtained based on intensity distribution imagesobtained by using the diffracted waves 220, 004, and 111, respectively(step S21 in FIG. 7). After each of the strain distribution images areacquired, the NBD method is used to quantify the diffracted waveintensity (step S22 in FIG. 7). For example, three particular pointsamong the strain distribution images are measured by the NBD method toexecute strain quantification of the diffraction intensity. As describedabove, the diffraction intensity and strain amounts can suitably beexpressed by straight-line approximation.

Next, shear strain is calculated based on the strain distributions inthe three directions (X, Y, and θ (35.3-degree) directions) (step S23 inFIG. 7). Shear strain γ_(xy) can be acquired by expression 3 in whichthe X-, Y-, and θ-direction strains are denoted by ε_(xx), ε_(yy),andε_(θ), respectively.

$\begin{matrix}{{\gamma_{xy} = \frac{ɛ_{\theta} - \left( {{ɛ_{xx}\cos^{2}\theta} + {ɛ_{yy}\sin^{2}\theta}} \right)}{\cos \; \theta \; \sin \; \theta}}{\theta = {35.3{^\circ}}}} & \left\lbrack {{expression}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In addition, expression 4 is obtained by transforming expression 3.Namely, the strain ε_(θ) can be calculated by using the measured strainε_(xx) and ε_(yy) and the calculated shear strain γ_(xy).

$\begin{matrix}{ɛ_{\theta} = {\frac{ɛ_{xx} + ɛ_{yy}}{2} + {\frac{ɛ_{xx} - ɛ_{yy}}{2}\cos \; 2\; \theta} + {\frac{1}{\gamma_{xy}}\sin \; 2\; \theta}}} & \left\lbrack {{expression}\mspace{14mu} 4} \right\rbrack\end{matrix}$

By selecting an appropriate direction θ for the strain ε_(θ), thedirection exhibiting a strain maximum level can be determined. Strain insuch direction is referred to as principal strain, and the direction ofthe principal strain can be determined by this calculation. Namely, inexpression 4, θ exhibiting a maximum strain amount ε_(θ) is thedirection of the principal strain, and the strain amount ε_(θ) in thatdirection is the magnitude of the principal strain. For example, asillustrated in FIG. 9, by representing and outputting the value and thedirection of principal strain at each point as the length and thedirection of an arrow, respectively, a stress source can be determined(step S24 in FIG. 7).

In addition, the shear strain γ_(θ) in an arbitrary direction θ can beobtained by expression 5 (step S25 in FIG. 7). If silicon/single-crystal is used, the Si (111) plane is a slip plane. Forexample, by obtaining a shear strain distribution image on this planewhen θ=54.7 degrees and −54.7 degrees, the possibility of occurrence ofa crystal defect can be predicted.

γ_(θ)=(ε_(xx)−ε_(yy))sin 2θ+γ_(xy) cos 2θ  [expression 5]

Namely, according to the second exemplary embodiment, the strain ε_(θ)in an arbitrary direction can be obtained by using quantified straindistributions in a plurality of directions and expressions 3 and 4.Thus, from the direction exhibiting a maximum strain level ε_(θ), themagnitude and the direction of principal strain can be obtained. Inaddition, from the magnitude and the direction of the principal strain,a stress source can be determined, as illustrated in FIG. 9.

In addition, the shear strain γ_(θ) in an arbitrary direction θ can beobtained based on expression 5. While the shear strain in asemiconductor device is a cause of a crystal defect, according to thesecond exemplary embodiment, the possibility of occurrence of a crystaldefect can be predicted.

Third Exemplary Embodiment

The calculation processing apparatus 200 in the evaluation system 10 inFIG. 1 is not necessarily a dedicated calculation processing apparatus.By causing a general-purpose computer such as an EWS or a PC to executea dedicated evaluation program stored in the storage apparatus 280, thegeneral-purpose computer, the electron microscope 100, and peripheralapparatuses such as the sample orientation control apparatus 140, thescanning coil and lens control apparatus 180, and the electron beamdetector 190 can be allowed to function as the evaluation system 10. Inthis case, peripheral apparatuses connectable to the general-purposecomputer can be used as the display apparatus 290, the input apparatus270, and the storage apparatus 280. In addition, by causing thegeneral-purpose computer to execute the evaluation program stored in thestorage apparatus 280, the general-purpose computer can be allowed tofunction as the calculation processing apparatus 200 including thecontrol unit 210, the strain distribution image extraction unit 221, thestrain quantification unit 230, and the stress analysis unit 240.Namely, according to the third exemplary embodiment, by causing acomputer to execute a dedicated program, a scanning transmissionelectron microscope and the computer controlling the scanningtransmission electron microscope and processing measurement dataobtained by the scanning transmission electron microscope are allowed tofunction as the evaluation system and to execute the evaluation methodaccording to the first and second exemplary embodiments.

EXAMPLE 1

Next, example 1 will be described. In example 1, the evaluation methodaccording to the first exemplary embodiment described with reference toFIGS. 1 to 4 is applied to a minute device having a trench gate asdisclosed in Patent Document 4, for example. FIGS. 5A and 5B illustratea diffracted wave and an intensity image thereof used for analyzingstrain in the X <110> direction, respectively. FIGS. 5D and 5Eillustrate a diffracted wave and an intensity image thereof used foranalyzing strain in the Y <001> direction, respectively. In example 1,since the lattice strain can be displayed depending on the straindirection as illustrated in FIGS. 5B and 5E, the strain state can begrasped easily. In addition, the diffracted wave intensity images asillustrated in FIGS. 5B and 5E can be displayed by stress analysisdiagrams as illustrated in FIGS. 5C and 5F. FIGS. 5C and 5F are stressanalysis diagrams based on results of a process simulation executedseparately from the measurement in FIGS. 5B and 5E.

As described above, according to each of the exemplary embodiments ofthe present disclosure, the lattice strain can be divided intodirectional components to be evaluated instantly, and the strain can bequantified by using the NBD method. In addition, based on the obtainedresults, by using a calculation processing apparatus, a shear straindistribution and a principal strain distribution, which cannot bemeasured by Patent Document 2 or 3 or Non-Patent Document 2, can beobtained from a strain amount at each point. As a result, thepossibility of occurrence of a crystal defect can be predicted from theshear strain distribution, and a stress source can be determined by theprincipal strain distribution.

Fourth Exemplary Embodiment

Next, an evaluation system and an evaluation method according to afourth exemplary embodiment will be described. In the fourth exemplaryembodiment, based on the principle of the present disclosure describedin the first to third exemplary embodiments and example 1, the straindirection separation performance (separation of the X-direction strainand Y-direction strain) is improved.

As illustrated in FIG. 10A, if stress is caused on the evaluation sample300 in a direction perpendicular thereto (in the X direction), an idealperpendicular-direction strain is caused. In this case, when the strainin the X-direction is evaluated, a target reciprocal lattice point onlymoves in the X direction, as illustrated in FIG. 10B. However, if theevaluation sample 300 is an actual LSI device, stress is caused incomplex directions, as illustrated in FIG. 11A. As a result, forexample, complex lattice strain including shear strain is caused. Thus,as illustrated in FIG. 11B, a target reciprocal lattice point used forevaluating a strain distribution in the X direction moves not only inthe X direction but also in the Y direction. Consequently, sincecomponents such as the strain in the Y direction and shear strain aresuperimposed on a lattice strain distribution image obtained based on atarget diffracted wave, an error is caused when the strain in the Xdirection is measured.

Thus, to improve the strain direction separation performance, variousstudies were conducted. As a result, it was found that if the electronbeam incident on the crystal sample (evaluation sample 300) has greaterparallelism, the strain distribution image exhibits a greater contrast.FIGS. 12A and 12B illustrate shapes of Ewald spheres formed in thereciprocal space when the electron beam incident on the crystal samplehas a large convergent angle and a small convergent angle (theillumination wave focused on the crystal sample has greaterparallelism), respectively. As illustrated in FIG. 12A, if the focusedelectron beam has a large convergent angle, since electrons are causedto be incident on the crystal sample in various directions, wider Ewaldspheres are formed in the reciprocal space. As a result, since averageexcitation errors are made uniform, the strain contrast is weakened.

Thus, the fourth exemplary embodiment proposes a rectangular movableaperture (rectangular aperture 121) as illustrated in FIGS. 13 and 14.FIG. 13 is a block diagram illustrating an overall configuration of asystem 10 a for evaluating a distribution of lattice strain on crystalmaterial according to the fourth exemplary embodiment. Elements of theevaluation system 10 a in FIG. 13 that are different from those of theevaluation system 10 according to the first exemplary embodiment in FIG.1 will be described. In the evaluation system 10 a in FIG. 13, anilluminating-system lens apparatus 120 a includes a rectangular aperture121 (an illumination aperture having a rectangular opening (aperture)).The rectangular aperture 121 has a rectangular opening (aperture), asillustrated in a plan view in FIG. 14A. FIG. 14B is a perspective viewillustrating an electron beam being incident on the evaluation sample300. As illustrated in FIG. 14B, the electron beam emitted from theelectron beam source 110 (see FIG. 13) is narrowed by the rectangularaperture (illumination aperture) 121 and is then focused on theevaluation sample 300 by the imaging-system lens apparatus 150 (see FIG.13). In addition, based on the evaluation system 10 a in FIG. 13, acontrol unit 210 a included in a calculation processing apparatus 200 acontrols the illuminating-system lens apparatus 120 a including therectangular aperture 121.

By including this rectangular aperture 121 in the illuminating-systemlens apparatus 120 a, the electron beam convergent angles in the X and Ydirections can be set to be asymmetrical to each other. As a result,diffraction intensity change can be obtained, focusing only the changeof a reciprocal lattice point in one direction.

The fourth exemplary embodiment will be described in more detail withreference to FIGS. 15A and 15B. FIG. 15A illustrates the relationshipbetween a cross section of the electron beam in the longitudinaldirection of the rectangular aperture and reciprocal lattice points (thesame as FIG. 12A). FIG. 15B illustrates an X-Y cross section of a dottedline portion in FIG. 15A (zero-order reciprocal lattice plane). Sincethe electron beam in the Y direction has a large convergent angle, theelectron beam is incident in various angles, and wider Ewald spheres areformed. However, since the electron beam in the X direction has greaterparallelism, no Ewald spheres appears on the zero-order reciprocallattice plane. As a result, while small excitation errors are exhibitedin the movement direction of the reciprocal lattice points (in the Ydirection), large excitation errors are exhibited in the otherdirection. Namely, since the electron beam has a large convergent anglein the Y direction, even if, of the focused electron beam, the electronbeam focused in one direction exhibits small excitation errors becauseof movement of the reciprocal lattice points in the Y direction, theelectron beam focused in the other direction exhibits large excitationerrors. Therefore, the diffraction intensity is not changed. However,since the movement in the X direction is due to the electron beam havinggreater parallelism, the diffraction intensity is changed. As a result,it is possible to obtain a lattice strain distribution having reducedstrain errors in the short-side direction of the rectangular aperture.

In this example, the rectangular aperture 121 is set so that the longand short side thereof are perpendicular and parallel to the directionof a certain diffracted wave detected with respect to a transmittedwave, respectively. For example, in FIG. 3B, when detecting thediffracted wave 220 diffracted in the X direction with respect to thetransmitted wave, the direction of the rectangular aperture 121 isadjusted so that the long and short sides thereof are parallel to theY-axis and the X-axis, respectively. Similarly, in FIG. 3B, whendetecting the diffracted wave 002 or 004 diffracted in the Y directionwith respect to the transmitted wave, the direction of the rectangularaperture 121 is adjusted so that the long and short sides thereof areparallel to the X-axis and Y-axis, respectively. In addition, while acertain advantageous effect can be obtained as long as the long side islonger than the short side; however, to reduce strain errors, it isdesirable that the ratio of the long side to the short side be 4:1 orgreater.

FIG. 16 illustrates a rectangular aperture 121 having a plurality ofrectangular apertures on a single metal plate. This rectangular aperture121 in FIG. 16 includes an X-direction evaluation rectangular aperture401, a Y-direction evaluation rectangular aperture 402, and a111-direction evaluation rectangular aperture 403. In this way, straindistribution images having greater strain separation performance in theX, Y, and 111 directions can be obtained. Generally, an illuminatinglens system includes a plurality of electron beam lenses, a deflector,and an aperture. Thus, by using an upstream lens and an aperture, arectangular aperture having the long side thereof in an arbitrarydirection can be selected.

FIG. 17 is a flow chart illustrating a method of evaluating adistribution of lattice strain on crystal material according to thefourth exemplary embodiment. When compared with the flow chart in FIG. 2illustrating the evaluation method according to the first exemplaryembodiment, the flow chart in FIG. 17 includes step S31 before thedetector 190 captures a certain diffracted wave in step S3. In step S31,the rectangular aperture 121 is adjusted, based on the certaindiffracted wave to be detected. The adjustment of the rectangularaperture 121 (step S31) may be executed at an arbitrary timing, as longas the adjustment is executed before the detector 190 captures a certaindiffracted wave in step S3. For example, if the rectangular aperture 121is a rectangular aperture including a plurality of apertures (openings)as illustrated in FIG. 16, when an upstream lens of theilluminating-system lens apparatus 120 a is controlled, an aperturematching a certain diffracted wave to be detected can be selected.Alternatively, the relative direction of the rectangular aperture 121and the evaluation sample 300 on the XY plane may be adjusted byrotating the rectangular aperture 121 and/or the evaluation sample 300on the XY plane, for example. Other steps similar to those according tothe first exemplary embodiment are denoted by the same referencecharacters, and repetitive descriptions will be omitted.

Needless to say, the above fourth exemplary embodiment can be combinedwith the analysis of a principal strain distribution and a shear straindistribution in an arbitrary direction according to the second exemplaryembodiment. According to the fourth exemplary embodiment, the straindirection separation performance can be improved. Thus, it is expectedthat a principal strain distribution or a shear strain distribution inan arbitrary direction can be analyzed more accurately. In addition, thefourth exemplary embodiment can be implemented by causing ageneral-purpose computer such as an EWS or a PC described in the thirdexemplary embodiment to execute a dedicated evaluation program. Thecomputer program caused to function as the evaluation system accordingto the fourth exemplary embodiment includes a program for controllingthe relative direction of the opening (aperture) of the rectangularaperture 121 with respect to the evaluation sample 300 in the electronmicroscope 100 a.

According to each of aspects, modes, or exemplary embodiments of thepresent disclosure, a strain distribution image is obtained by selectinga certain diffracted wave. Thus, by evaluating a selected diffractedwave, a strain distribution image per direction can be obtained. Inaddition, the possibility of occurrence of a crystal defect can bepredicted or a stress source can be determined, based on such straindistribution image per direction. Modifications and adjustments of theexemplary embodiments and examples are possible within the scope of theoverall disclosure (including the claims and the drawings) of thepresent invention and based on the basic technical concept of thepresent invention. Various combinations and selections of variousdisclosed elements (including the elements in the claims, exemplaryembodiments, drawings, etc.) are possible within the scope of the claimsof the present invention. That is, the present invention of courseincludes various variations and modifications that could be made bythose skilled in the art according to the overall disclosure includingthe claims and the drawings and based on the technical concept.

1. A method of evaluating a distribution of lattice strain on crystalmaterial, the method comprising: illuminating a sample having a crystalstructure with an electron beam in a zone axis direction (termed as“illuminating step”) and selectively detecting a certain diffracted wavediffracted in a certain direction among a plurality of diffracted wavesdiffracted by the sample (termed as “selectively detecting step”); andrepeating said illuminating step and said selectively detecting stepwhile scanning the sample, and obtaining a strain distribution image ina direction corresponding to the certain diffracted wave fromdiffraction intensity at each point of the sample (termed as “straindistribution image obtaining step”).
 2. The method according to claim 1,further comprising steps of: repeating said strain distribution imageobtaining step for each of a plurality of diffracted waves diffracted indirections different from each other and obtaining strain distributionimages in directions corresponding to the directions of the diffractedwaves, based on the diffracted waves diffracted in directions differentfrom each other; and executing stress analysis on the sample, based onthe strain distribution images in the plurality of directions.
 3. Themethod according to claim 2, further comprising: quantifying themagnitude of strain of the strain distribution image.
 4. The methodaccording to claim 3; wherein executing the stress analysis furthercomprises: calculating shear strain in a certain direction, based onstrain measurement results in a plurality of directions; and calculatingthe magnitude and direction of principal strain, based on strainmeasurement results in a plurality of directions and the calculatedshear strain in a certain direction.
 5. The method according to claim 3;wherein executing the stress analysis further comprises: calculatingshear strain in a certain direction, based on strain measurement resultsin a plurality of directions; and obtaining shear strain in an arbitrarydirection, based on the strain measurement results in a plurality ofdirections and the calculated shear strain in a certain directions. 6.The method according to claim 1; wherein the sample is a TEM samplewhich is formed by thinning material having a crystal structure to havea uniform thickness, and the certain diffracted wave is obtained byforward scattering.
 7. The method according to claim 1; wherein, in saidselectively detecting step, the electron beam with which the sample isilluminated is narrowed to a rectangle having long and short sidesperpendicular and parallel to a direction in which the certaindiffracted wave is diffracted with respect to the transmitted wave,respectively, and the narrowed electron beam is focused on the sample.8. A system of evaluating a distribution of lattice strain on crystalmaterial, the system comprising: a scanning transmission electronmicroscope that illuminates a sample with an electron beam, scans thesample, and detects a diffracted wave transmitted or diffracted by thesample; and a strain distribution image extraction unit that selects acertain diffracted wave among diffracted waves transmitted or diffractedby the sample, and obtains a strain distribution image.
 9. The systemaccording to claim 8, further comprising: a strain quantification unitthat quantifies strain intensity of the strain distribution image. 10.The system according to claim 8, further comprising: a stress analysisunit; wherein the strain distribution image extraction unit obtainsstrain distribution images in a plurality of directions eachcorresponding to the direction of one of a plurality of diffracted wavesdiffracted in directions different from each other; and wherein thestress analysis unit executes stress analysis on the sample, based onthe strain distribution images in a plurality of direction obtained bythe strain distribution image extraction unit.
 11. The system accordingto claim 10; wherein the stress analysis unit is configured to executeprocesses of: calculating shear strain in a certain direction, based onthe strain distribution images in a plurality of directions; andcalculating the magnitude and direction of principal strain, based onthe strain distribution images in a plurality of directions and thecalculated shear strain in a certain direction.
 12. The system accordingto claim 10; wherein the stress analysis unit is configured to executeprocesses of: calculating shear strain in a certain direction, based onthe strain distribution images in a plurality of directions; andcalculating shear strain in an arbitrary direction, based on the straindistribution images in a plurality of directions and the calculatedshear strain in a certain direction.
 13. The system according to claim8; wherein the scanning transmission electron microscope furthercomprises a sample orientation control apparatus that adjusts crystalorientation of the sample with respect to the direction of the incidentelectron beam.
 14. The system according to claim 8, further comprising:a display apparatus that displays evaluation results including thestrain distribution image.
 15. The system according to claim 8; whereinthe scanning transmission electron microscope comprises an illuminationaperture having a rectangular opening to narrow the electron beam andilluminate the sample with the narrowed electron beam; and wherein therectangular opening of the illumination aperture can be set so that thelong and short sides thereof are perpendicular and parallel to adirection in which the certain diffracted wave is diffracted withrespect to the transmitted wave, respectively.
 16. The system accordingto claim 15; wherein the illumination aperture comprises a plurality ofrectangular openings, each of which has long and short sides indifferent directions; and wherein, in accordance with a direction inwhich the certain diffracted wave is diffracted, an opening isselectable from the plurality of openings as an aperture to illuminatethe sample with the electron beam with the selected opening.
 17. Anon-transitory computer-readable recording medium storing a computerprogram used in an evaluation system comprising a scanning transmissionelectron microscope and a computer that controls the scanningtransmission electron microscope and processes measurement data obtainedby the scanning transmission electron microscope, the computer programcausing the computer to execute processes of: controlling the scanningtransmission electron microscope so that a sample having a crystalstructure is illuminated with an electron beam and a diffracted wavetransmitted or diffracted by the sample is detected; and scanning thesample, selecting a certain diffracted wave among diffracted wavestransmitted or diffracted by the sample, and obtaining a straindistribution image.
 18. The non-transitory computer-readable recordingmedium according to claim 17; wherein said scanning transmissionelectron microscope comprises an illumination aperture having arectangular opening whose direction is changeable; and wherein saidprocess of controlling the scanning transmission electron microscopeincludes controlling of the direction of the rectangular opening of theillumination aperture in accordance with a direction in which thecertain diffracted wave is diffracted with respect to the transmittedlight.